1. Field of the Invention
The present invention relates to filter circuits, and more particularly filter circuits using stacked spiral inductors.
2. Description of the Background Art
Printed circuit (PC) boards typically include multiple dielectric layers having metal deposited on surface regions, the dielectric layers being laminated together. Conductive metal regions on the outer dielectric layers of the PC boards may include large metal regions to provide ground planes for signal lines on other layers, and may further be etched to provide lines for carrying signals.
The outer dielectric layers of the PC board typically support lumped elements such as capacitors or inductors. Although inner dielectric layers do not typically support such lumped components, conductive metal regions may be etched on the inner dielectric layers to form elements such as inductors and capacitors. Inductors can be created by forming a line in a spiral pattern, while capacitors can be created using two metal regions separated by a gap.
To take advantage of self coupling between windings of an individual spiral inductor, as well as mutual coupling between windings of separate spiral inductors, spiral inductors may be stacked. U.S. Pat. No. 5,656,849, entitled xe2x80x9cTwo-Level Spiral Inductor Structure Having A High Inductance To Area Ratioxe2x80x9d by Burghartz et al., discloses stacking spiral inductors in layers of an integrated circuit device to form an inductor which occupies minimal space and takes advantage of the added mutual inductance between the stacked inductors.
The present invention utilizes multiple conductive layers separated by dielectric material in a PC board to provide a filter including stacked spiral inductors.
The present invention further utilizes spiral inductors provided between layers of a PC board in combination with lumped element capacitors, inductors, or a combination of capacitors and inductors provided on an external surface of the PC board to create a tunable filter which occupies a minimal amount of space.
The present invention includes stacked spiral inductors with an external end of each of the spiral inductors coupled by a via to openings in ground planes provided on external surfaces of the PC board. Components such as capacitors, inductors, or combinations of capacitors and inductors can be used to couple the vias connected to the external ends of the spiral inductors to one or more of the ground planes. The lumped element components can be attached to outer dielectric layers of the PC board and altered to enable tuning of the filter after manufacture of the PC board. The lumped elements can also be connected from the external end of one spiral inductor to the external end of another spiral inductor to create additional filter configurations.
Internal ends of the spiral inductors are further connected together by a via. The internal ends of each of the spiral inductors can further be connected to ground planes provided on outer surfaces of the PC board. With the internal ends of the stacked spiral inductors connected by a via to a ground plane, superb grounding is provided which can significantly limit ground bounce noise relative to filters with components connected in a planar fashion which have longer lines connecting filter components to ground.
Metallization can be provided on layers of the PC board between spiral inductors to alter the coupling between the spiral inductors and provide alternative filter configurations. In one such configuration, a simple transmission line is provided on a layer between the spiral inductors. One end of the transmission line is connected to a via connecting the internal ends of the spiral inductors, while a second end of the transmission line is connected by a via to ground plane regions on external layers of the PC board. Such a transmission line will be inductively coupled to the spiral inductors and will provide isolation between the spiral inductors and ground.
The spiral inductors can further be located to overlap to maximize mutual coupling, or located to not overlap to reduce mutual coupling to provide alternative filter structures. In one configuration, a first pair of spiral inductors are positioned to overlap, a second pair of spiral inductors are further positioned to overlap, while the first and second pairs do not overlap. Such a structure provides a filter made up of two connected transformers.